Embodiments of the present invention pertain to integrated circuits. Specifically, embodiments of the present invention pertain to a circuit that can provide a sub-band gap reference voltage with operating supply voltage less than the band gap potential.
Many contemporary CMOS (complementary metal-oxide silicon) integrated circuit chips contain a large digital core along with some peripheral analog circuitry. The analog circuitry typically includes reference voltage circuits that are relied upon by various analog blocks and/or by select digital circuits. These reference voltage circuits should optimally provide a stable, dependable and accurate reference voltage.
One of the most widely adopted reference voltage circuits is referred to as a xe2x80x9cband gapxe2x80x9d circuit. The band gap circuit is based on an established physical phenomenon exhibited by silicon. Basically, silicon has a band gap potential of 1.21 volts. The band gap potential of silicon can be exploited to produce an extremely reliable and tight reference voltage.
According to the prior art, in order to produce a band gap reference voltage of 1.21 volts, an operating supply voltage of 1.5 volts or greater is typically required in order to provide a margin of overhead. The majority of analog CMOS circuits today operate at a voltage of three (3) volts, which amply meets the needs of conventional bandgap circuits. However, advances in technology that have resulted in smaller and faster digital circuitry are pushing analog counterparts to keep pace. This, combined with a desire to reduce the voltage and current (i.e., power) requirements, is pushing analog circuitry to operate at voltages as low as one (1) volt, and perhaps even less than 1 volt. Quite obviously, this is less than the bandgap potential of 1.2 volts. As such, current bandgap circuits are not adequate in light of the desire to reduce the operating supply voltage to below the bandgap potential. Accordingly, what is needed is a circuit that can provide a stable reference voltage with an operating supply voltage less than the bandgap potential.
Embodiments of the present invention pertain to a circuit that provides a stable reference voltage with an operating supply voltage less than the bandgap potential and also less than a zero-bias threshold voltage. In one embodiment, the sub-bandgap circuit includes an operational amplifier having an N-well input stage and a proportional to absolute temperature (PTA) current source having a forward-biased P-bulk.